The phrase "to mount a serious challenge" means to make a strong and determined effort to compete or oppose someone or something with a high chance of success. Full definition
You feel that's the only thing stopping us from mounting a serious challenge this season. (justarsenal.com)
But then again it's the champions league so it comes as no surprise, we never even mount a serious challenge we just make up the numbers. (justarsenal.com)
This always happens after a poor performance or game, especially at this stage of a season and when it seems like the Gunners have let another campaign slip by without mounting a serious challenge for the Premier League title. (justarsenal.com)